In conventional timepieces, a gear train is formed of a center wheel having an elastic portion which is held and supported by a center wheel arbor.
When time is corrected, the center wheel is sheared and run idle through the center wheel arbor, thereby preventing trasnmission of rotation of the gear train from being transmitted to second hand (fourth wheel and pinion). Further, in a setting mechanism, a clutch wheel is moved by a winding stem, a setting lever, a yoke, and so on. The clutch wheel is fit into the members of gear train, and then time correcting is carried out by external operating materials.
However, a conventional timepiece structure has the following problems since a sharing machanism utilizes the elastic portion of the center wheel in order to correct time.
1: The center wheel shearing torque is unstable. When shearing torque is so high, wheel members moving for time correcting, such as a center wheel pinion, a minute wheel, and a clutch wheel tend to be worn and broken.
Further, since the initial state of second wheel shearing torque is relatively high, the above members moving for time correcting must be formed of high hardened metallic materials (hardeded carbon steel), thereby causing high cost from the viewpoints of process and materials. Plastic materials and the molding technique have been recently improved and a gear train structure made of all plastic materials has been investigating. However, it was impossible to realize the gear train made of all plastic members as far as a gear train made of a shearing mechanism is adopted.
2: When the center wheel shearing torque is so low, a minute hand is individually rotated by fine vibration, light impact or the like in normal operating condition, namely the normally operating condition in which the winding stem is pushed into the inside of the timepiece (hereinafter referred to as the normal operating condition). As a result, time tends to be wrong.
3: In a setting mechanism, a clutch wheel must be made of metallic materials in order to ensure the intensity.
When time is corrected, a clutching wheel must be engaged with a winding stem as an external control member at the angle cutting portions thereof to transmit the rotation. In the normal operation condition, a clutch wheel must be removed from the engagement with gear train members.
Therefore, metallic materials such as a setting lever and a yoke are required in a conventional timepiece in order to move a clutch wheel.
This invention solves such problems and an object of the invention is to obtain smooth time correcting mechanism without a shearing mechanism formed of wheels by removing one part of gear train members when time is corrected.
Another object of the invention is to provide a timepiece improved in rationality by utilizing a plastic material for not only a gear train mechanism but also a setting mechanism instead of conventional requirement such as a setting lever, a yoke or the like.